This research program will study the response of the rat kidney to fractionated irradiation. This study intends to provide information on the radiation tolerance of the kidney that will aid in the planning of optimal radiation therapy schedules. Specifically, it will determine how the radiation tolerance of the kidney changes with fractionation, determine whether there are early indicators of kidney damage that correlate with late kidney damage, and determine whether non-lethal kidney damage is repaired between treatment courses 6 to 18 months apart. Information will also be gained concerning the slope of the dose-response curve for late radiation damage, and concerning optimal radiotherapy schedules. Rats will receive bilateral kidney irradiations in several fractionation schedules, ranging from single doses to 32 fractions in 72 days. These irradiations are performed both with and without pentobarbital anesthesia. Animals will be followed for at least 18 months, with periodic determinations of blood pressure, blood chemistry, urine chemistry, and iodohippurate clearance times. Kidney doses will range from those producing only slight nephritis to those that will cause death within 12 months. Some of the animals surviving the first course of treatment will be given a second fractionated treatment 6 to 18 months later.